Conventional air conditioning systems rely upon traditional refrigerator cycles in which electrical energy and conventional energy sources power the cooling and conditioning action of a refrigerant through compression and expansion phases of the refrigeration cycle. The rapid spread of conventional residential and commercial air conditioning has created excessive peak load demand during summer days on utility systems particularly in southern climates. As a result, attention has turned to the feasibility of air conditioners which rely on alternative energy sources. Desiccant air conditioning systems are able to utilize alternative energy sources such as solar energy as the ultimate energy source for cooling and air conditioning thereby reducing electric power consumption and reliance upon conventional power sources.
Air conditioning by evaporative cooling has previously been restricted to hot dry climates such as found in the Southwest. Desiccant systems now promise extension of the "swamp cooler" principle of evaporative chilling to hot and humid summer air such as, for example, found in the Southeast and eastern portions of the country. Present desiccant solar air conditioners are described for example in the article, Lindsley, "Solar Air Conditioners", Popular Science, July 1984, pp. 64 et seq.
These prior art desiccant solar air conditioners work as follows. During the cooling mode or adsorption cycle, hot humid air enters the intake side of the air conditioning system and passes through one side of a slowly turning desiccant wheel or circular desiccant bed. Water vapor and other moisture vapor is adsorbed on the extended desiccant material surface area, drying the air and releasing the latent heat of condensation. The hot dry air from the desiccant bed wheel then passes through a heat exchanger such as an air to air heat exchanger wheel giving up some of the heat to an exhaust air stream. The air is then reconditioned to have parameters in the desired comfort zone by passing through an evaporative element or unit where moisture is evaporated back into the air, for example by spraying, cooling the air to a desired temperature and humidifying the air to a desired relative humidity.
As the desiccant wheel or other desiccant bed is saturated to capacity with moisture, it is regenerated while rotating through the desorption cycle. Warm air or heated air typically from the attic passes through a solar actuated heat exchanger for further heating to a desired regeneration temperature. Hot fluid circulates in the solar heat exchanger coil from solar collectors. The solar heat exchanger is also referred to as the regeneration coil from which the heated air then passes back through the other side of the rotating desiccant bed or desiccant wheel removing the moisture and drying the desiccant for reuse. A conventional fuel source such as a gas heater may be used as a backup source of energy for recharging or regenerating the desiccant in the absence of adequate solar energy. Typically the system is also available for operation as a heating system in winter by using another solar heating coil or heat exchanger with the solar collectors and blowing solar heated air to appropriate circulating ducts.
A number of disadvantages are attendant upon previous desiccant solar air conditioners. A primary limitation of prior art systems is that temperature change is limited to that available from transformation of sensible heat in the air to latent heat of moisture contained in the air, and transformation of latent heat of moisture back to sensible heat. Thus, air temperature conditioning is subject to the limitations inherent in the substantially closed system of heat exchange between the latent and sensible forms of heat energy. Furthermore, reduction in temperature is associated with increase in humidity frequently above desirable levels of relative humidity, severely limiting simultaneous control of both temperature and relative humidity to desirable levels. As a result it is difficult to achieve air conditioning for domestic or commercial use within desired comfort zones. The close inverse relationship of temperature and humidity in conventional air conditioning systems particularly limits the independent control of humidity which must always increase for a reduction in temperature. As a result a useful desiccant solar air conditioner for the hot and humid southeastern regions of the country still does not exist.